1. Field of the Invention
The present invention relates to an X-ray exposure apparatus, an X-ray exposure method used in forming a fine structure such as a semiconductor device, the fine structure and the semiconductor device fabricated using the same. More particularly, the present invention relates to an X-ray exposure apparatus and an X-ray exposure method capable of suppressing a pattern transfer defect caused by secondary electrons from a substrate even if X-rays with wavelengths shorter than an absorption edge wavelength of a substrate are used in a transfer technique for a fine pattern such as a semiconductor integrated circuit and the like, the fine structure and the semiconductor device.
2. Description of the Background Art
FIG. 81 is an illustration of a construction showing a general X-ray exposure system (Japanese Patent Laying-Open No. 2000-338299). In FIG. 81, as a radiation source, a radiation generator (synchrotron radiation (SR) apparatus) 101 with a critical wavelength of 0.846 nm is adopted, which emits radiation 102. The radiation 102 is reflected twice by mirrors 103a and 103b with an oblique incidence angle of 1 degree into beams with a rectangular section and transmitted through a beryllium window 104 with a thickness of 20 xcexcm. A radiation path including the X-ray mirrors and the beryllium window is called a beam line 105.
The radiation coming out from the beam line 105 is directed, for illumination, to a mask 108 constituted of an absorber 107 and a membrane 106 which is a transmissive part. A fine pattern is determined by placement of absorbers on membrane 106 of the transmissive part. The X-rays pass through the mask and are thereby patterned. The patterned X-rays impinge on a resist 110 coated on a substrate 109 and illuminated parts are photochemically activated to form a resist pattern. X-rays are absorbed by the resist, but all of them is not absorbed; the X-rays remaining unabsorbed are transmitted through the resist to reach substrate 109 and impinge on it.
FIG. 82 is a graph showing spectra of X-rays with which surfaces of resists are illuminated in two kinds of X-ray exposure apparatuses. In FIG. 82, a spectrum having a peak in the neighborhood of a wavelength of 0.43 nm is used as exposure X-rays having a peak in the shorter wavelength range. On the other hand, a spectrum having a peak in the neighborhood of a wavelength of 0.7 nm is used as exposure X-rays in the longer wavelength range. Exposure X-rays in the longer wavelength range have a spectrum at the surface of a resist, obtained in a case where an SiC mirror as a reflecting X-ray mirror is used and an SiC film of a thickness 2 xcexcm is adopted in a transmissive part of a mask. Exposure X-rays in the shorter wavelength range have a spectrum at the surface of the resist, obtained in a case where a nickel mirror is used and a diamond film of a thickness 2 xcexcm is adopted in the transmissive part of a mask.
In prior art X-ray exposure, X-rays in the longer wavelength range equal to or more than an absorption-edge wavelength of silicon of 0.7 nm were mainly employed. In contrast, in exposure using X-rays in the shorter wavelength range, X-rays including wavelengths equal to or less than 0.7 nm and down to the order of 0.3 nm were employed.
In an X-ray proximity exposure technique, there has remained the following problem, which has been one of reasons for difficulty in exposure using X-rays in the shorter wavelength range. That is, in exposure on a silicon substrate or the like, a problem arises that a fogging effect in a resist at the bottom caused by photoelectrons (secondary electrons) from the silicon substrate is enhanced as the wavelength range of exposure X-rays is shifted to the shorter wavelength side. For this reason, none of wavelengths equal to or shorter than 0.67 nm, which is an absorption-edge wavelength of silicon, has been able to be employed.
However, microfabrication of a semiconductor device and others has progressed with certainty, in which case, X-rays with short wavelengths, if they could be employed, would be, with no doubt, advantageous in miniaturization of a semiconductor device. This is because blurring of a pattern effected by diffraction or the like is suppressed more with shorter wavelength X-rays. Hence, in order to avoid the fogging effect caused by photoelectrons from the silicon substrate in a case of X-rays in the shorter wavelength range than the absorption-edge wavelength of silicon, the following schemes have been employed: a 2-layer resist scheme or a scheme forming a coating on a surface of a silicon substrate prior to resist coating.
With the 2-layer resist scheme or the scheme of forming a coating on a surface of a silicon substrate prior to resist coating, however, a problem has occurred that man-hours increase, reducing production efficiency. In light of such a problem, development has been desired of an exposure technique using X-rays with a shorter wavelength than the absorption edge of silicon without reducing production efficiency as compared with the prior art transfer technique for a fine pattern.
It is an object of the present invention to provide an X-ray exposure method and an X-ray exposure apparatus capable of using exposure X-rays of short wavelengths advantageous for formation of a fine pattern by suppressing a fogging effect due to secondary electrons from a substrate, not limiting to a silicon substrate, enhanced in company with use of the exposure X-rays of short wavelengths; and a fine structure and a semiconductor device using the same.
An X-ray exposure method of the present invention is an exposure method including the steps of: forming, by coating, a resist film on a substrate made of a material having an absorption-edge in and near an illumination wavelength range; and illuminating the resist film with X-rays in the wavelength range including an absorption-edge wavelength through an X-ray mask. In this X-ray exposure method, exposure is performed at a ratio of an amount of X-rays absorbed in a surface layer down to a prescribed depth of the substrate to an amount of X-rays absorbed in the resist film being equal to or less than a prescribed value.
Since an amount of generated secondary electrons is proportional to an amount of absorbed X-rays, with this construction of the X-ray exposure method, generation of secondary electrons from the substrate is suppressed while ensuring sufficient photochemical activation in the resist to thereby enable photochemical activation in the resist with less of a fogging effect. Note that the passage xe2x80x9can amount of X-rays absorbed in a surface layer down to a prescribed depth of the substratexe2x80x9d means an amount of X-rays absorbed in a depth range from the surface to a depth of the maximum shooting range of secondary electrons generated in the substrate. While secondary electrons are generated in the substrate in proportion to the X-ray amount, the behavior of the generated secondary electrons can be approximated such that a half thereof directs to the resist film side while the other half directs to the opposite side thereto.
In the above X-ray exposure method of the present invention, exposure is performed providing means reducing X-ray intensity in a wavelength range of an absorption spectrum to which the absorption-edge of the material of the substrate belongs in an optical path leading to the substrate.
According to the method, since X-rays in a wavelength range in which high absorption by the substrate occurs are cut off, an absorbed amount of X-rays in the resist relatively increases as compared with that in the substrate. Hence, since an amount of generated secondary electrons that increase in proportion to an absorbed amount of the X-rays in the substrate is suppressed, a fogging effect at the bottom of the resist caused by secondary electron from the substrate can be suppressed. As a result of the suppression, exposure X-rays including the absorption-edge wavelength of a material of the substrate can be used, thereby, enabling suppression of a fog in a fine pattern or the like defect caused by X-ray diffraction and so on.
In the X-ray exposure method, the resist film can include an element having the absorption spectrum overlapping an absorption spectrum to which the absorption-edge of the material of the substrate belongs.
According to the construction, it is enabled to increase an amount of X-rays absorbed in the resist film, while decreasing an amount of X-rays reaching the substrate. As a result, a fogging effect in the resist caused by secondary electrons generated in the substrate can be suppressed. An element included in the resist film in this case is not necessarily required to have an absorption spectrum having an absorption-edge therein. The element included in the resist film has only to have an absorption spectrum overlapping the wavelength range.
In the X-ray exposure method, the resist film includes an element an absorption-edge of which is in a wavelength range longer than the absorption-edge of the material of the substrate and there is provided means for reducing X-ray intensity mainly in a wavelength range equal to and shorter than the absorption-edge wavelength of the material of the substrate.
According to the X-ray exposure method, the resist film can be illuminated with exposure X-rays in a wavelength range hard to be absorbed by the substrate but easy to be absorbed in the resist film. Hence, the resist film can be photochemically activated into a prescribed fine pattern while suppressing an amount of generated secondary electrons from the substrate.
In the X-ray exposure method, the resist film includes an element an absorption-edge of which is in a wavelength range shorter than the absorption-edge of the material of the substrate and there is provided means for reducing X-ray intensity mainly in a wavelength range equal to and longer than the absorption-edge wavelength of the element included in the resist film.
According to the construction, as described above, the resist film can be illuminated with exposure X-rays in a wavelength range hard to be absorbed by the substrate but easy to be absorbed in the resist film. Hence, the resist film can be photochemically activated into a prescribed fine pattern while suppressing an amount of generated secondary electrons from the substrate. In this case, since X-rays in a wavelength range shorter than the absorption-edge wavelength of the material of the substrate is absorbed in the resist film to suppress a fogging effect caused by diffraction or the like, the method is advantageous for formation of a fine pattern.
In the X-ray exposure method, there can be provided at least one of a filter and a transmissive part of the mask with an absorptive power mainly in and near a wavelength range of the absorption-edge wavelength of the material of the substrate.
According to the construction, by including an element having an absorption-edge wavelength different from an absorption-edge wavelength of the material of the substrate, an amount of X-rays absorbed in the resist is increased, which enables suppression of an amount of X-rays absorbed in the substrate.
In the X-ray exposure method, the at least one of a filter and a transmissive part of a mask can decrease transmission of X-rays of wavelengths equal to and shorter than the absorption-edge wavelength of the material of the substrate.
In this case, by including an element having an absorption wavelength range longer than an absorption-edge wavelength of the material of the substrate in the resist, absorption of X-rays in the substrate can be suppressed while performing sufficient photochemical activation in the resist.
In the X-ray exposure method, the at least one of a filter and a transmissive part of a mask can decrease transmission of X-rays of wavelengths equal to and longer than the absorption-edge wavelength of the material of the substrate.
In this case, by including an element having an absorption wavelength range shorter than the absorption-edge wavelength of the material of the substrate in the resist, absorption of X-rays in the substrate can be suppressed while performing sufficient photochemical activation in the resist.
In the X-ray exposure method, an reflecting X-ray mirror capable of changing a reflecting spectrum while holding an optical axis at a fixed position is provided in a radiation path in the radiation source side with respect to the X ray mask and by adjusting a position of the X-ray reflecting mirror, X-ray intensity in an wavelength range of an absorption spectrum to which the absorption edge of the material of the substrate belongs can be reduced.
According to the construction, by adjusting a position of the X-ray reflecting mirror while holding the optical axis at the fixed position, the resist film can be illuminated with X-rays easy to be absorbed in the resist film excluding a wavelength range easy to be absorbed in the substrate. Modulation of an intensity spectrum of exposure X-rays by the reflecting X-ray mirror can be achieved to an great extent in terms of a wavelength range and an intensity, both.
In the X-ray exposure method, an element performing a major absorption among elements included in the resist film can be an element having a mass number larger than that of an element constituting the substrate.
An element having a larger mass number has a larger absorptive power at a smaller content thereof. Hence, a content of the element included in the resist can be reduced.
In the X-ray exposure method, there is provided a resist including an element having an absorption-edge wavelength; and as an exposure X-ray, such an X-ray is used that is in a wavelength range from said absorption-edge wavelength to a wavelength shorter by 400 eV than said absorption-edge wavelength.
By including a material having an absorption edge in a wavelength range shorter than an absorption-edge wavelength of the material of the substrate, not only is a fogging effect caused by secondary electrons from the substrate is suppressed, but a fine pattern can also be transferred with more of sharpness. Furthermore, by narrowing a wavelength range width of X-rays, more than more of sharpness can be achieved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.